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Scientific question: Influence of terrain heterogeneity

Evolution of Wind, Temperature and Humidity Structure in the Valley is discussed below:

Hypotheses: While the assumption of an idealized two-dimensional valley within two parallel ridges, as in Perdigão, is a reasonable first step in modeling, the natural variability of topography and land use can greatly modify the flow within the valley and over the ridges. In particular, the presence of a simple “gap” in one ridge can generate secondary circulation, jetting, interacting shear layers and cross-slope flows that modify the flow and turbulence over a certain spatial extent – which is determined by the overall topography, topographic anomalies (gaps), approach flow and stability.

Approach:

  1. Characterize the approach flow, background stability and orography of the gaps;

  2. Verify Jackson & Hunt (1975) framework for low slope angles (i.e. h/L < 0.1) and no gap areas of the fore mountain at neutral stability and explain discrepancies;

  3. Map 3D velocity and turbulence fields in the vicinity of the gap and away from it at high resolution, identify and explain the differences;

  4. Measure the pressure field, local circulation, separated flow, flow structures, secondary circulation and turbulence at selected locations at high space-time resolution;

  5. Measure coherent structures at the ridge shear layers and gap-separated flows and estimate related momentum transports

  6. Identify the length scales of flow distortions, both vertical and horizontal, at the gap and away from it,

  7. Quantify internal wave radiation under stable conditions and upslope flow separation under unstable conditions at gap area and away from it as a function of flow and stratification parameter

  8. Study how the results of (a–f) depend on atmospheric stability (stable, unstable or neutral).


Implications: Topographic inhomogeneities of microscales are known to substantially modify both thermally and synoptically driven flows (Rotach & Zardi 2007), which cannot be captured by mesoscale models. The spatial extent and magnitude of this modification, as well as possible unsteady phenomena such as vortex shedding, is of importance in wind turbine siting and operations (Fesquet et al. 2009).